Dr Daryl Howard, a co-author of
the study, told BBC News: "I think what is really exciting is that we have now
been able to add one more Degas artwork for the world to see."

It had long been known that
Degas' portrait of a woman wearing a black bonnet and dress, which he painted in
the late 1870s, covered an earlier painting.

A ghostly impression of the
composition appears as a dark stain on the sitter's face, and over the years has
become more prominent as the oil paint thinned.

Conventional X-rays revealed the
outline of another image was lurking beneath, but without scraping away the
outer painting, the researchers required a much more powerful technique to show
any detail.

For that, they used the
Australian Synchrotron, a huge accelerator that generates more powerful X-rays,
to peer beneath the top layers of paint.They were able to detect the
metallic elements in the pigments that Degas had used in his underlying
artwork.

Dr Howard, from the Australian
Synchrotron, said: "Each element has its own unique signature, and so that gets
collected. "And what we do is analyse that
data and build up these 'elemental maps'. And that allows us to image all the
different pigments used in the panting."

Through this they were able to
see in colour and in remarkable detail Degas' hidden work: a portrait of a woman
with auburn hair.

Unfinished, it shows how the
artist made several attempts to rework her features. But the scientists said it was
clear enough to try to work out who the subject was.

Dr Howard explained: "Once the
image had come through, basically what I did was to look up Degas's catalogue of
works. And I would say in under five minutes, it seemed that we had a good
match.

"We suggested that the hidden
portrait is of a model he has painted several times before - Emma Dobigny. I
think the likeness is quite amazing."

Of particular note are her ears,
which are pointed and pixie-like - reminiscent of early Degas
paintings.

The researchers believe the
portrait was left for several years before it was painted over.

"Previous academic works about
Degas suggests it was around 1869 when he was painting Emma Dobigny," Dr Howard
said.

"The current portrait is thought
to have been painted about seven to 10 years later, so there is a big gap. It is
possible that the painting remained in his studio for several years before he
decided to paint over it."

Michael Varcoe-Cocks, head of
conservation at National Gallery Victoria where the artwork is currently on
display, commented: "It's always an exciting moment to behold something produced
by the hand of an artist that was otherwise previously unseen.

"The situation is slightly
different here as the image has always been partially visible in the second
painting, so the process was more like revealing the other side of a half-drawn
curtain."

Mr Varcoe-Cocks said the artist's
choice to abandon a portrait and recycle the canvas was a common
practice.

"What is unusual is that instead
of painting out the earlier form, he partially included its features in the
second composition and this tells us something about his inquisitive and
non-conventional approach to art."However the art expert said it was too early
to establish the identity of the underlying portrait, and that curators at
National Gallery Victoria were carrying out further research to date the hidden
image and confirm the sitter.

He added that there was no
question that the underlying image would remain hidden.

"We respect Degas' choices, and
honour his final composition for a unique work in its own right," said Dr
Varcoe-Cocks. "It is unquestionably the work
the artist was happy with, as it is the completed and signed painting."
The researchers say this X-ray
technique, which is non-destructive, is a crucial tool for examining
artworks.

"In the past, if a conservator
wanted to know more about a painting they would have to take a chip off of it
and analyse that small chip… and of course, that is another little bit of the
painting gone," said Dr Howard."With our technique, we can scan
the whole painting and give it back to the gallery in the same condition that
they gave it to us."